1. Field of the Invention
The present invention relates to a high speed tool steel, a material for a blade edge, a cutting tool, and a manufacturing method of the material for the blade edge, the high speed tool steel being suitable as a source material for the material for the blade edge of the cutting tool such as a saw blade used in cutting a metal.
2. Description of Related Art
Conventionally, in cutting a metallic material such as steel, a cutting tool is used, a typical example of which is a saw blade such as a band saw and a circular saw. The saw blade is manufactured in general by the following process. First, a source material such as a steel ingot or billet is prepared by casting a molten steel adjusted to a predetermined component composition, or a source material is prepared by hot pressure molding a powder obtained from the molten steel using an atomizing method, and the source material is subjected to hot working. Thereafter, various machining processes and heat treatments are performed and a material for a blade edge having a flat wire shape and the like is manufactured. Next, the material for the blade edge is welded to a body material by electron beam welding or laser welding or the like; the resulting product is subjected to sharpening, then quenching and tempering, and is finished to a saw blade as a final product. As the source material for the above material for the blade edge, a high speed tool steel SKH59 specified in JIS G 4403 (corresponding to HS2-9-1-8 specified in ISO4957) is widely used. SKH59 has excellent red hardness and is a source material with excellent cutting durability, and has excellent characteristics as the source material for the material for the blade edge of the saw blade. For example, Japanese Patent Laid-Open Publication No. 2010-280022 discloses an invention of a band saw blade and a manufacturing method thereof in which SKH59 is adopted as a source material for a material for a blade edge.
Since a saw blade having a blade edge manufactured using SKH59 has excellent cutting durability, the saw blade is used in cutting various steels. However, it is known that, depending on a condition of use, early wear, chipping and the like of the blade edge can occur as forms of wear and breakage limited to the blade edge. As a countermeasure against these forms of wear and breakage, improvements in the design of the blade edge, such as a shape, hardness and surface treatment of the blade edge, have been carried out. However, chipping can still occur at an early stage. Other than matters related to the design of the blade edge, one of the reasons for the above chipping to occur is that coarse carbides are contained in a structure of the material for the blade edge. That is, when a large amount of significantly coarse carbides are contained in the structure of the material for the blade edge (for example, when a large amount of carbides having an absolute maximum length of 25 μm or more are contained in a cross-sectional structure), these significantly coarse carbides remain in the structure of the blade edge even after quenching and tempering, and toughness of the blade edge decreases. A stress (fracture stress) required to break the blade edge during use decreases, and fracture originating from a coarse carbide occurs. Therefore, reducing the size of the carbides in the structure of the material for the blade edge is effective in inhibiting chipping.
The component composition of SKH59 that allows a high degree of hardness to be realized is an alloy design in which a large amount of carbides are formed in a structure. In a case of a high speed tool steel having such a component composition, when a source material such as a steel ingot or billet is formed, significantly coarsened massive eutectic carbides are likely to be formed in a cast structure of the source material. In general, the eutectic carbides (M2C) in the cast structure have plate-like shapes, and can be changed to granular carbides (M6C) by hot working. However, when the eutectic carbides are in a significantly coarse massive form, in a manufacturing process of the material for the blade edge, even by successive hot working (wire rod processing), the carbides cannot be changed to be sufficiently granular, so that, in an annealed structure of the material for the blade edge, there is a large amount of the significantly coarse carbides having an absolute maximum length of 25 μm or more. The carbides that cannot be finely formed in the annealed structure do not become fine even after welding and sharpening, nor in the quenching and tempering of a final process. As a result, this becomes a main reason why a saw blade containing a large amount of coarse carbides in a blade edge structure has excellent wear resistance, but chipping resistance of the saw blade deteriorates.
However, for the material for the blade edge according to the conventional technology, reducing the size of the carbides in a structure is not necessarily easy. The reason is that, in order to achieve efficiency in an actual operation, it is necessary to increase a weight of the source material such as steel ingots or billets so that a cooling rate slows down when molten steel solidifies, and thus undesirable coarse eutectic carbides are formed in a cast structure of the source material after solidification and remain until a subsequent process. A purpose of the present invention is to provide a high speed tool steel that maintains excellent characteristics of SKH59 and for which carbides can be finely formed in a solidified structure even at an actual operation level, and to provide a material for a blade edge that is made using the high speed tool steel, and a cutting tool. Another purpose of the present invention is to provide a manufacturing method of the material for the blade edge.